In relation to a transistor structure of a semiconductor device, a further reduction in the microscopic size of an insulated gate type field effect transistor (e.g., MOSFET) using a silicon oxide film has been advanced. As for a technique for improving performance of an N-type MOS transistor as well as a P-type MOS transistor in the field effect transistor, there has been proposed a dual stress liner (DSL) technique (see, Patent Document 1). In the DSL technique, a SiN (silicon nitride) film having a tensile stress is formed over a region where the N-type MOS transistor is formed, to thereby exert a tensile stress on that region, and a SiN film having a compressive stress is formed over a region where the P-type MOS transistor is formed, to thereby exert a compressive stress on that region.
As shown in FIG. 1, a plurality of transistors 103, each including a gate electrode 101 and sidewalls 102 formed on side surfaces of the gate electrode 101, is provided on a surface of an Si semiconductor wafer (hereinafter, referred to as “wafer”). As shown in FIG. 2, a SiN film 104 is formed by a CVD method or the like so as to cover the transistors 103. Next, the SiN film 104 is etched to have a desired thickness as shown in FIG. 3. The SiN film 104 having a desired thickness serves as a stress film for exerting a stress on the region where the transistors 103 are formed. Due to the presence of the SiN film 104, a tensile stress or a compressive stress can be exterted on the region where the transistors 103 are formed.
As for a method for etching a SiN film, a reactive ion etching (RIE) or a plasma etching is widely used. However, the RIE or the plasma etching may inflict damages on a film other than the SiN film on the wafer.
Patent Document 1: Japanese Patent Application Publication No. 2008-288364
Therefore, the present inventor has found, as a method for forming a SiN film having a desired thickness with less damages by precisely controlling an etching amount, a method for etching a SiN film, which serves as a stress film of strained transistors, with high precision by using a gas containing a halogen element such as a hydrogen fluoride gas or the like. In accordance with the method for etching the SiN film by using the gas containing the halogen element, an etching amount can be easily controlled and a SiN film having a desired thickness can be obtained. Further, since the etching is performed by chemical reaction with the SiN film, physical damages caused by a plasma on a film other than the SiN film may be reduced.
However, the present inventor has discovered from the various experiments that the method for etching the SiN film by using the gas containing the halogen element is disadvantageous in that an etching amount is decreased compared to the RIE or the plasma etching and also in that an etching amount distribution is non-uniform in the surface of the wafer. If the etching amount distribution is non-uniform, characteristics of the respective transistors are changed.